Median aerodynamic diameters

FIGURE 5.28 Estimated overall airway deposition as a function of initial particle size and particle hygroscopicity for particles with mass median aerodynamic diameters (MMAD) between 0.1 and 10 p.m. ° Geometric dispersion, a measure of particle size distribution, principally affects only smaller MMAD,... 

Activity Median Aerodynamic Diameter (AMAD)—The diameter of a unit-density sphere with the same terminal settling velocity in air as that of the aerosol particle whose activity is the median for the entire size distribution of the aerosol. 

Based on 46 measurements, the activity median aerodynamic diameter of Pb-212 averaged 0.13 pm (0g = 2.97), while Pb-214 averaged 0.16 pm (Og = 2.86). The larger median size of Pb-214 was attributed to a-recoil depletion of smaller aerosols following decay of aerosol-associated Po-218. 

Aerodynamic Size Distributions of Naturally-Radioactive Aerosols. Measurements of radionuclide distributions using cascade impactors indicate that Be-7 and Pb-210 are associated with larger aerosols than Pb-212 and Pb-214 (Robig et al., 1980 Papastefanou and Bondietti, 1986). Measurements of Pb-210 associations over oceans indicated activity median aerodynamic diameters (AMAD) near 0.6 pm (Sanak et al., 1981). The impactor measurements of Moore et al. (1980) on Pb-210, Bi-210, and Sr-90 sizes in continental air indicated that about 80% of the activity from all three nuclides was associated with aerosols below 0.3 pm. That work also determined that the mean age of aerosol Pb-210 was about a week. Knuth et al. (1983) compared Pb-210 and stable Pb sizes at a continental location and found that 78% of the Pb-210 found below 1.73 pm was smaller than 0.58 pm. Young (1974) reported that the most of the Be-7 in the atmosphere was associated with submicron aerosols. 

Pb-212 vs S0%- LPI Distributions. Figure 3 presents a summary of the average Pb-212 AMADs and SOj MADs (mass median aerodynamic diameters) determined from a series of LPI measurements made during the period January to October, 1985. The Pb-212 data were derived from collections made at the same time as S0jj and from measurements made to compare Pb-212 vs Pb-214. The mean aerodynamic diameter of Pb-212 was about three times smaller than SoJ . Much less sulfate was found in the aerosol fraction below 0.08 um, compared with Pb-212. While Pb-212 was largely absent above 0.52 um, about 20% of the SoJ occurred above this size. 

Table III summarizes the median aerodynamic diameters of Pb-210, Be-7, and S0j found in measurements made through March 1986. Be-7 distributions are substantially smaller than Sojj , regardless of the time of the year. The Pb-210 data, while limited, suggests that summer sizes are larger than winter sizes.

Table II. Summary of Mean Monthly Activity Median Aerodynamic Diameters (AMAD) and Geometric Standard Deviations (tfg) of Radon and Thoron Daughter Size Distributions in Ambient Aerosols...

Table III. Median Aerodynamic Diameters (MAD) and Geometric Standard Deviations (Og) of Pb-210 and Be-7 Based on Radioactivity and SO Measured with High-Volume Cascade Impactors...

The aerodynamic size distributions of Pb-214, Pb-212, Pb-210, Be-7, P-32, S-35-SoJ , and stable SO4 were measured using cascade impactors. Pb-212 and Pb-214, measured by alpha spectroscopy, were largely associated with aerosols small than 0.52 11m. Based on over 46 low-pressure impactor measurements, the mean activity median aerodynamic diameter (AMAD) of Pb-212 was found to be 0.13 11m, while for Pb-214 the AMAD was larger—0.16 lim. The slightly larger size of Pb-214, confirmed with operationally different impactors, was attributed to a-recoil-driven redistribution of Pb-214 following decay of aerosol-associated Po-218. A recoil model was presented that explained this redistribution. Low-pressure impactor measurements indicated that the mass median aerodynamic diameter of SoJ ... 

Published results on the concentration and size distribution of small particles in mainstream smoke vary widely, with concentrations ranging from 107 to 1011 cm-3 and with NMAD (number median aerodynamic diameter) ranging from 0.2 to 0.7 fim (Ishizu et a/., 1978). The MMAD (mass median aerodynamic diameter) of undiluted mainstream smoke particles ranges between 0.93 and 1.00 finl (Langer and Fisher, 1956 Holmes et a/., 1959). Lower values of the MMAD for diluted mainstream smoke, which decreased with degree of dilution, are reported by Hinds (1978). However, the particle size distributions for mainstream smoke appear to have little relevance to its retention and distribution in the lung, for reasons discussed below. Note that the concentration of tars in mainstream smoke is about 1,000 times that of air in smoke-filled rooms. 

Fig. 1. Deposition of inhaled particles of different sizes (mass median aerodynamic diameters) in the three regions of the respiratory tract. Each shaded area indicates the variability of deposition when the aerosol distribution parameter, o, (geometric standard deviation) was varied from 1.2 to 4.5. The assumed tidal volume was 1450 cm3. (Reproduced from Health Physics, vol. 12, pp. 173-207,1966 by permission of the Health Physics Society).

Activity median aerodynamic diameter h Geometric standard deviation. 

The various pharmacopeias outline appropriate methods for aerodynamic assessment of partiele size distribution. The USP defines the size distribution through mass median aerodynamic diameter (MMAD) and geometrie standard deviation (GSD). 

Both from deposition studies and force balances it can be derived that the optimum (aerodynamic) particle size lies between 0.5 and 7.5 pm. Within this approximate range many different subranges have been presented as most favourable, e.g. 0.1 to 5 pm [24], 0.5 to 8.0 pm [25], 2 to 7 pm [26] and 1-5 pm [27-29]. Particles of 7.5 pm and larger mainly deposit in the oropharynx [30] whereas most particles smaller than 0.5 pm are exhaled again [31]. All inhalation systems for drug delivery to the respiratory tract produce polydisperse aerosols which can be characterized by their mass median aerodynamic diameter (MMAD) and geometric standard deviation (oq). The MMAD is the particle diameter at 50% of the cumulative mass curve. 

Used to derive a chronic inhalation Minimal Risk Level (MRL) of 2 x 10" mg/m nickel for soluble nickel salts dose adjusted for intermittent exposure (6/24 hours, 5/7 days), multiplied by the Regional Deposited Dose Ratio (0.9714 for pulmonary region deposition mass median aerodynamic diameter [MMAD] = 2.5 pm, sigma = 2.4 pm), and divided by an uncertainty factor of 30 (3 for extrapolation from animals to humans, and 10 for human variability). 

The half-life of nickel in the lungs of rats exposed by inhalation has been reported to be 32 hours for nickel sulfate (mass median aerodynamic diameter [MMAD] 0.6 pm) (Hirano et al. 1994b), 4.6 days for nickel subsulfide ( Ni3S2 activity, median aerodynamic diameter [AMAD] 1.3 pm), and 120 days for green nickel oxide ( NiO, AMAD 1.3 pm) (Benson et al. 1994). Elimination half-times from the lung of rats of 7.7, 11.5, and 21 months were calculated for green nickel oxide with MMADs of 0.6, 1.2, and 4.0 pm, respectively (Tanaka et al. 1985, 1988). 

The Mass Median Aerodynamic Diameter (MMAD) was reported as a lower limit of 0.8 pm and an upper limit of 1.2 pm for an average of 1.0 pm (pg 33 Hartman 1990). The Geometric Standard Deviation (GSD) was reported as a lower limit of 1.2 pm and an upper limit of 1.5 pm for an average of 1.35 or 1.4 pm. The Regional Deposited Dose Ratio (RDDR) from Table HI under the ER (Extrarespiratory effects) column is... 

A study in dogs indicates that absorption of inhaled metallic silver particles with a median aerodynamic diameter of approximately 0.5 pm is extensive, and is not dependent upon particle size (Phalen and Morrow 1973). Absorption was measured in one dog that remained anesthetized during the entire period between exposure and sacrifice. In this dog, 3.1% (0.8 pg) of the deposited material was dissolved, transported out of the lungs, and was found mostly in liver and blood 6 hours after exposure a 1 pg/cm /day absorption rate for metallic silver was estimated by the authors. Up to 90% of the deposited silver was estimated to be absorbed into the systemic circulation based on all experimental data. Clearance from the lung to the blood was triphasic, with half-lives of 1.7, 8.4, and 40 days. 

Minimal data are available from typical inhalation studies in laboratory animals to allow evaluation of extent or dose-dependency in inhaled arsenic absorption. Beck, Slayton and Farr (2002) reported a study in which rabbits were exposed to 0.05, 0.1, 0.22, or 1.1 mg m-3 of arsenic trioxide 8 hours/day, seven days/week for eight weeks. The particle size (mass median aerodynamic diameter, MMAD) ranged from 3.2 to 4.1pm. On the basis of minimal elevation of inorganic arsenic in plasma until exposure levels were at or above 0.22 mg m-3, the authors concluded that systemic uptake of arsenic trioxide following inhalation exposure was low and did not contribute significantly to body burden until relatively high levels of exposure were achieved. 

Aerodynamic diameter Particles suspended in gases typically have irregular shapes. The aerodynamic behavior of an irregularly shaped particle may be modeled with an idealized sphere of uniform composition. The diameter of the sphere is the aerodynamic diameter (compare with mass median aerodynamic diameter). 

Mass median aerodynamic diameter A measure of particle size related to its mass. The aerodynamic diameter of an aerosol, where 50% of the mass of the aerosol has particles with larger aerodynamic diameters and 50 % has smaller diameters. 

A mean value of 3 x 10-10 m-1 for Kr was deduced by Shinn et al. (1983) from measurements of airborne Pu over a bare field near the Savannah River Processing Plant. Here the activity median aerodynamic diameter was 3 //m. The main reason why Kr measured in Nevada and North Carolina was much lower than Kr measured at Maralinga and Monte Bello is the effect of ageing on the characteristics of the deposited material. 

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